A power converter is a power processing circuit that converts an input voltage or current source into a specified output voltage or current. In off-line applications wherein power factor correction, total harmonic distortion (THD) reduction and a stable, regulated voltage are desired, the power converter typically employs a boost converter.
The power converter generally includes an electromagnetic interference (EMI) filter, coupled to a source of alternating current (AC) power. A rectifier, coupling the EMI filter to the boost converter, rectifies the AC power to produce an unregulated DC voltage. The boost converter receives the unregulated DC voltage and generates therefrom a controlled, high DC voltage. A DC/DC converter, coupled to the boost converter, then converts the high DC voltage (e.g., 400 VDC) to a lower voltage (e.g., 48 VDC or 24 VDC).
A conventional boost converter generally includes an inductor, coupled between a source of input voltage (e.g., the rectifier that provides the unregulated DC voltage) and a power switch. The power switch is then coupled in parallel with a rectifying diode and an output capacitor. The output capacitor is usually large to ensure a constant output voltage to a load (e.g., a DC/DC converter). The output voltage (measured at the load) of the boost converter is always greater than the input voltage.
For high AC input voltages, in conjunction with the output voltage of a boost converter being greater than the input DC voltage, the output of the conventional boost converter may be too high for commonly available semiconductor devices. For three phase, high AC input voltages, a so-called "split" boost converter that provides two equal output voltages, which are lower than the input voltage, has been suggested to accommodate semiconductor devices rated for the output voltages. Separate DC/DC converters are then used with each output.
Switched-mode power converters generally suffer from EMI noise problems. Power converters, therefore, must be designed to meet domestic and international EMI regulatory requirements. A high switching frequency (e.g., 100 kHz) of the power switches is a major source of EMI. The input EMI filter shields the source of AC power from the EMI generated by the power switches.
Split-boost converters may also contain EMI noisy outputs. One way to filter the switching frequency and thereby obtain EMI quiet outputs is to add an output EMI filter, consisting of an inductor and a capacitor. The output EMI filter normally has high Q characteristics due to practical design considerations. High Q filters, however, are difficult to damp without incurring substantial losses. It would therefore be preferable to eliminate the need for such filters.
Another problem encountered with the split-boost converter is balancing the output voltages. Prior to the closing of the power switches, the output voltages must be close in value to guard against a large circulating current from developing and possibly destroying the power switches. Additionally, if the power switches are not opened or closed simultaneously, the power switch that is closed first or opened last suffers greater turn-on and turn-off losses, respectively, and the switching losses are not evenly distributed between the power switches. It is difficult to predict which power switch suffers the greater switching loss because the switching timing is affected by uncontrollable factors, such as the threshold of the gate drive circuit. As a result, the thermal design of the converter becomes more difficult.
Accordingly, what is needed in the art is an improved DC/DC converter topology that mitigates or substantially eliminates the above-described problems.